1. Field of the Invention
This invention relates to systems and methods for synchronizing a 3D video projector.
2. Description of the Related Art
Two dimensional video projectors have been widely available for many decades. Recently, three dimensional (3D) stereoscopic video projectors have been developed that produce flickerless 3D stereoscopic images in part by increasing the video projector's operating frequency from approximately 60 frames per second (FPS) to approximately 85 FPS and beyond. This higher frame rate is important because rates below 85 FPS can give the impression of excessive flickering to the viewer.
As shown in FIG. 1, in order to view the images from a 3D stereoscopic video projector 12 connected to an image source 11, special optical devices may be used and/or special eyewear may be worn by the viewer, such as a pair of liquid crystal (LC) shutter-based glasses 13. Such glasses 13 include shutters 14A, 14B that cover the user's eyes and alternately transition between clear and opaque via a shutter control signal, generally referred to as 22. A video signal 20 is delivered from the image source 11 to the video projector 12 which then controls the production and delivery of the images 18, 19 synchronously with the shutters 14A, 14B so that only the left eye is presented a left eye image 18 and the right eye is presented a right eye image 19.
As shown in FIG. 2, other 3D stereoscopic video projector systems are found in the prior art that utilize a polarization flipping device 16 to alternate the polarization of light exiting the video projector 12. An example of a polarization flipping device is one sold by Real D, Inc. of Beverly Hills, Calif. under the trademark Z SCREEN. The video projector 12 is connected to an image source 11, and projects an image (designated 18, 19) through a polarization flipping device 16 and onto a polarization preserving screen (not shown) which is watched by a viewer wearing passive polarized eyewear 17. The image source 11 sends a flipping device control signal 22′ to control the flipping device 16.
Recently, semiconductors, called digital light processors or digital micro-mirror devices (DMDs) (Texas Instruments, Inc. sells a digital light processor under the trademark DLP) have been developed to manipulate light digitally. DMDs act as light switches consisting of up to and beyond 1.3 million microscopic mirrors, each of which is able to tilt back and forth up to 5,000 times per second. In 3D stereoscopic video projector systems the movement of the DMD is coordinated with the projector's light source, and with the video or graphic signals transferred to an eye-switching device to produce seamless images to the viewer. Today, 3D video projectors with three DMDs are relatively common with a single DMD being assigned to one of three critical colors. Unfortunately, such 3D video projectors are relatively expensive.
Another type of 3D video projector utilizes a single DMD. While 3D video projectors with a single DMD are less expensive than 3D video projectors with three DMDs, they also employ a color wheel divided into several color segments which filter the white light from the projector's light source into specific colors which are passed in succession onto the single DMD. A color sequencing firmware, (hereinafter referred to as firmware) is also employed to control the operation of the single DMD.
3D video projector systems used with LC shutter glasses 13 must use an eye-switching signal 22 to synchronize the eyewear's shutters 14A, 14B. This eye-switching signal 13 is typically generated externally from the video projector, for instance at the image source 11 as shown in FIG. 1. Thus when used with single DMD projectors, the eye-switching signal 22 is indirectly synchronized with the projector's color wheel. As the color wheel spins during operation, the eye-shutters on the glasses 13 quickly alternate to create transparent and opaque periods. Unfortunately, the transition between the transparent and opaque periods is not instantaneous which reduces the overall quality of the image seen by the viewer. For example, when the transition period occurs during the Red color segment, some of the time normally required to complete a full Red duty cycle may be lost, causing the Red color in the image to appear weak. It is especially problematic when presenting human faces which appear noticeably pale or lifeless. This transition either between transparent and opaque periods or between opaque and transparent periods shall be referred to as the “unstable transition phase”. While adjustments could be made to the video projector to move the problem from one color segment to another, the problem, heretofore, has not been resolved.